Orthopedic implants include numerous structures that are implanted into a living body in order to augment or supplant the bone structure of the patient into whom it is implanted. For example, such implants can include plate structures that act to stabilize broken bones during the healing process. They can also include vertebral cages or vertebral inter-body spacers that maintain the relative position of the vertebrae during fusion. Implants can also include constructs such as mesh that is used to maintain fragments in position or rod systems that are used to align and position the vertebrae. The present invention can be used with any such implant so long as it includes a portion having a through bore adjacent a bottom surface that opposes the bone to which the implant is anchored. In general, although not necessarily exclusively, this portion will have a bottom bone engaging surface, and an opposing top surface (top and bottom being defined relative to the bone surface that engages the implant) defining a thickness in between to form a plate like area which includes the threaded through bore.
One problem that may be encountered with implants that are fastened to the bone using a screw is that there may be too much play between the screw and the implant, or that the screw may back out of the implant. Of course, the implant forms part of a dynamic system with the skeletal structure, and is subject to constant and varied forces. Locking mechanisms help to ensure that the screw does not back out of the plate. If a screw backs out of an implant depending on the placement there may be a risk that it may project beyond the top surface of the implant into a sensitive biological area. This raises the possibility of irritation to adjoining soft tissue. Another possibility if there is too much movement between the plate and the screw is that the plate may play against the screw and increase the risk of shearing the screw.
Various solutions have been used to lock the screw relative to the implant. One prior art solution has been to provide the screw with a locking member such as a nut or collet that engages a portion of the screw. An additional solution has been to provide the screw with a second set of locking threads near the head of the screw. These locking threads mate with internal threads within the screw hole of the recess. However, the existence of multiple start threads can cause problems with cross threading of the male threads within the female threads of the recess during implantation. This can cause the screw to become irreparably stuck in the implant, able to travel neither into nor out of the bore. While this problem would be at least frustrating in the context of a static bracket such as for example, a wall anchor, the surgical context and difficulties in accessing the site, as well as the time concerns increase the complications with such problems considerably. Often a surgeon is working a substantial distance, like 5-15 inches through an incision to the bone implantation site, the area is cramped, visibility is obscured, and timing is critical.
The present invention provides a very elegant solution to the locking problem which enjoys a minimum of component parts, and which neatly locks the screw in the implant while avoiding prior art problems of cross threading of the locking threads with the mating locking threads of the plate.